A number of biologically active peptides or proteins, including hormones, cytokines, neuropeptides and growth factors, are initially generated in the form of larger, inactive precursor peptides. These precursor peptides, or propeptides, including prohormones and profactors, generally require specific intracellular proteolytic processing to turn them into their active forms for biological functions [1, 2]. In terms of protein manufacturing, the precursor forms of the peptides are often first synthesized instead of the mature forms. This is because the mature forms of the peptides often have complex conformations, low expression yield, or are structurally unstable. In terms of protein drug delivery, the propeptides, but not the mature peptides, are linked to another protein moiety through chemical conjugation or recombinant fusion to achieve specific delivery goals and enhance overall protein stability [3]. Therefore, in order to exhibit biological activity, the propeptides need to be processed and activated, which is an important and challenging step in the production of recombinant therapeutic proteins.
Conventional methods of delivering the prodrugs generally involve chemical conjugation to link together the propeptide with a delivery protein. However, the major obstacle for chemically conjugating the two domains is that the composition and size of the final product can be heterogeneous, which is unacceptable for therapeutic use. Therefore, there still exists a need for a better approach to form fusion proteins that link together a delivery domain with a prodrug domain.